1. Field of the Invention:
The present invention relates to a vacuum exhaust system, for example, for use in a semiconductor manufacturing process. More particularly, the invention relates to a vacuum exhaust system capable of exhausting a gas at a relatively large flow rate from a vacuum treatment chamber or the like.
2. Description of the Related Art:
Conventionally, a semiconductor manufacturing apparatus or a liquid crystal manufacturing generally comprises a vacuum chamber for performing etching or CVD processing or the like, and a vacuum pump for evacuating a process gas from the vacuum chamber and reducing the pressure in the vacuum chamber to a desired value. In general as a roots vacuum pump which has an ultimate pressure in a middle vacuum region is used as the vacuum pump. When the higher vacuum level is required, a turbo vacuum pump such as a turbo molecular pump is used as a main pump, and a vacuum pump such as a roots vacuum pump which has an ultimate pressure in a middle vacuum region is used as an auxiliary pump. The auxiliary pump is disposed downstream of the main pump and designed to evacuate the main pump until the back pressure of the main pump becomes a permissible value or lower. The main pump and the auxiliary pump are connected with each other by piping, and a necessary valve device is arranged in the piping. The turbo vacuum pump includes a turbo molecular pump and a molecular drag pump, which have an ultimate pressure in a ultra-high vacuum region and cannot evacuate a gas directly to the atmospheric pressure.
The auxiliary pump is normally disposed near the main pump, but may be installed away from it, or on a different floor. The exhaust speed (L/min) of the auxiliary pump, now generally selected, is such that the ratio of the exhaust speed (L/sec) of the main pump to the exhaust speed (L/min) of the auxiliary pump is about 0.2 to 1.0. The auxiliary pump is relatively large in size and high in cost.
Piping as a connection between the main pump and the auxiliary pump is usually thick piping with an inner diameter of Ø 40 mm or more, if the auxiliary pump is located away from the apparatus unit or on a different floor and the piping is long. Even if the auxiliary pump is located near the apparatus unit and the piping is short, piping with an inner diameter of Ø 25 mm or more is used to connect the main pump and the auxiliary pump.
With such a vacuum exhaust system, the diameter of the piping and the performance of the auxiliary pump (especially, the exhaust speed) are determined by the flow rate of process gas, the length of the piping, and the permissible back pressure of the main pump. Generally, however, the permissible back pressure of the main pump means merely that under the back pressure conditions, a continuous run is possible (rated speed can be maintained without issue of an alarm). If a wide range turbo-molecular pump is assumed as a main pump, for example, it actually occurs that exhaust performance at a low back pressure cannot be maintained before the back pressure reaches the permissible back pressure. A detailed description will be offered later on with reference to FIG. 6.
When a pump is used in the actual semiconductor manufacturing apparatus or liquid crystal manufacturing apparatus, therefore, a sufficient margin for the permissible back pressure of the main pump must be left so that full exhaust performance can be done. Consequently, the diameter of the piping increases.
In an exhaust system using piping having such a large diameter, if the piping is long, the piping itself occupies an expensive space in the clean room, increasing the costs of plant equipment. In the case of conventional large-diameter piping, straight pipes 4a , 4a of a length adapted for on-site conditions and an elbow 4b constituting a bend portion B are prepared and welded beforehand, as shown in FIG. 21B. This requires components such as the elbow 4b , increasing the procurement cost and handling cost. Furthermore, advance inspection of the workshop is necessary, and large expenses have been involved for operations in addition to welding work itself. For large diameter piping, a bending tool such as a bender cannot be used. Even if the tool is usable, the resulting piping poses a problem about strength. The same is true when a flexible tube is used for piping to be assembled at the workshop.
The present invention has been accomplished to solve the foregoing problems. An object of the invention is to provide a vacuum exhaust system which can realize cost reduction through a saving in entire space, simplification of piping construction work, and so on.
A first aspect of the present invention is a vacuum exhaust system comprising a vacuum chamber, means for introducing a gas into the vacuum chamber, a main pump for exhausting the vacuum chamber and reducing a pressure of the vacuum chamber to a desired pressure, an auxiliary pump disposed downstream from the main pump, and piping for connecting them, wherein an outer diameter of connecting piping as a connection between the main pump and the auxiliary pump is xc2xd inch (12.7 mm) or less, and a length of the connecting piping and capability of the auxiliary pump are combined so that a back pressure of the main pump becomes 5 Torr or more.
The background for establishment of the concept of this invention will be described. The relationship among the pressure before and after the connecting piping, the diameter of the piping, and the length of the piping is generally expressed by the following equation (1), provided that the connecting piping is a straight pipe.                                           π            128                    ·                                    D              4                                      η              ⁢                              xe2x80x83                            ⁢              L                                ·                                                    P                1                2                            -                              P                2                2                                      2                          =        Q                            (        1        )            
where Q: flow rate (Paxc2x7m3/s) of gas introduced into vacuum chamber
D: inner diameter (m) of piping
L: length (m) of piping
P1: back pressure (Pa) of main pump
P2: pressure (Pa) at inlet of auxiliary pump P2=Q/S if the exhaust speed of the auxiliary pump is S (m3/s) 
xcex7: coefficient of viscosity (Paxc2x7s) of gas introduced into vacuum chamber
Assume that an 8-inch wafer is etched in the vacuum treatment chamber. When an N2 gas is flowed at a maximum flow rate, the main pump back pressure calculated from the equation (1) is as shown in Table 1, provided that the length of the piping is set in consideration of the installation conditions and the conventional inner diameter is 25 mm or 40 mm. In this case, 2.0 Torr or more is sufficient as the permissible back pressure of the main pump.
Table 2 shows the results of calculation made when the inner diameter of the piping is set at a small value of Ø 10 mm under the same conditions so that on-site piping assembly work can be done. On-site piping assembly refers to a method by which pipes are bent at a site, where the apparatus has been or will be installed, by use of a pipe bending tool such as a bender to complete piping at the site. For example, assume that a vacuum chamber 1, piping 2, and a main pump 3 of a vacuum exhaust system are installed on an upper floor, while an auxiliary pump 5 is installed on a lower floor, as shown in FIG. 20. In this case, a bend portion B is created on the spot by means of a bending tool, such as a bender, to complete piping, as shown in FIG. 21A. Such an operation involving bending on the spot is necessary in most cases even when the components of the system are arranged on the same floor. For on-site piping completion, a flexible tube can be used if the outer diameter of connecting piping is xc2xd inch (12.7 mm) or less.
The results of Table 2 show that when the auxiliary pump is installed near the apparatus with the piping length of 2 m therebetween, a main pump having a permissible back pressure of about 5 Torr is necessary to downsize the piping to an inner diameter of Ø 10 mm or less. When the auxiliary pump is installed apart from the apparatus on the same floor, or the apparatus and the auxiliary pump are installed upstairs and downstairs, and the piping length is 5 m and 20 m, respectively, a main pump having a permissible back pressure of about 7 Torr and 15 Torr, respectively, is necessary to downsize the piping to an inner diameter of Ø 10 mm or less.
The present invention is based on the above findings, and comprises a combination of a piping path having a piping diameter made so small as to enable on-site piping assembly, and a vacuum pump which can be run while maintaining a necessary back pressure of the main pump anticipated for the small diameter piping.
A second aspect of the invention is a vacuum exhaust system comprising a vacuum chamber, means for introducing a gas into the vacuum chamber, a main pump for exhausting the vacuum chamber and reducing a pressure of the vacuum chamber to a desired pressure, an auxiliary pump disposed downstream from the main pump, and piping for connecting them, wherein an outer diameter of connecting piping as a connection between the main pump and the auxiliary pump is a value which enables the connecting piping to be assembled by on-site piping bending, and a length of the connecting piping and capability of the auxiliary pump are combined so that a back pressure of the main pump becomes 5 Torr or more.
A third aspect of the invention is a method for constructing a vacuum exhaust system comprising a vacuum chamber, means for introducing a gas into the vacuum chamber, a main pump for exhausting the vacuum chamber and reducing a pressure of the vacuum chamber to a desired pressure, an auxiliary pump disposed downstream from the main pump, and piping for connecting them, wherein connecting piping as a connection between the main pump and the auxiliary pump is assembled by on-site piping bending.
In the vacuum exhaust system or the method for constructing a vacuum exhaust system, the main pump may have a blade exhaust portion composed of moving blades and stationary blades arranged alternately, and at least a part of the blade exhaust portion may be constituted as a diametrical blade exhaust portion having projections and depressions formed in at least one of opposed surfaces of the moving blades and stationary blades.
The use of such a new type of turbo-vacuum pump makes it possible to perform a stable exhaust action even relatively high back pressure conditions. The main pump may be of a wide area type having a thread groove exhaust portion in addition to the blade exhaust portion.
A valve element for covering an inlet openably and closably, and a valve drive mechanism for opening and closing the valve element may be provided integrally with the main pump. With this construction, a single valve device can concurrently function as an opening/closing valve (gate valve) and an opening regulating valve (APC valve). Thus, an exhaust system around the chamber can be constituted compactly.
A heater for raising the temperature of piping may be provided at an arbitrary position of a piping portion which connects the main pump and the auxiliary pump. With the vacuum exhaust system of the invention, the pressure between the main pump and the auxiliary pump is high compared with a conventional vacuum exhaust system. Thus, an exhaust gas tends to build up as a solid product between the main pump and the auxiliary pump. To prevent clogging or the like of the piping, it is effective to raise the piping temperature to the temperature corresponding to the saturation vapor pressure of the exhaust gas or a higher temperature.
Similarly, at least one of a cooling trap and a heating trap for removing the product may be provided between the main pump and the auxiliary pump. By so doing, before the exhaust gas accumulates in the piping as a solid product, the exhaust gas is forcibly cooled to solidification, or is converted into other substance by a thermochemical reaction, whereafter the resulting matter can be removed.
According to the various aspects of the present invention described above, the exhaust system is composed of a combination of a piping path having a piping diameter made so small as to enable on-site assembly of piping, and a vacuum pump which can be run while maintaining a necessary back pressure of the main pump anticipated for the small diameter piping. This construction can decrease the space occupied by the piping itself, and simplify piping assembly work, thereby cutting down on the cost of the vacuum exhaust system as a whole.